The invention relates to a reactor for carrying out autothermal gas-phase dehydrogenations using a heterogeneous catalyst configured as a monolith and also a process using the reactor.
Ceramic or metallic monoliths have become established as catalyst supports for noble metal catalysts in mobile and stationary offgas purification. The channels offer a low resistance to flow and at the same time uniform accessibility to the outer catalyst surface for gaseous reaction media. This is advantageous compared to disordered beds in which a large pressure drop results from numerous deflections in the flow around the particles and the catalyst surface may not be uniformly utilized. The use of monoliths is generally of interest for catalytic processes having high volume flows under adiabatic reaction conditions at high temperatures. In chemical process technology, these features apply particularly to dehydrogenation reactions which occur in the temperature range from 400° C. to 700° C.
Progress in catalyst technology has made selective combustion of the dehydrogenation hydrogen in the presence of hydrocarbons possible, as described, for example, in U.S. Pat. No. 7,034,195. Such a mode of operation is referred to as autothermal dehydrogenation and allows dehydrogenation reactors to be heated directly, so that complicated apparatuses for indirect preheating and intermediate heating of the reaction mixture become unnecessary. One such process is described, for example, in US 2008/0119673. However, this process has the serious disadvantage that the dehydrogenation is carried out over a heterogeneous catalyst in pellet form: the high flow resistance of beds of pellets requires a large reactor cross section and a correspondingly low flow velocity in order to limit the pressure drop in the catalytically active layer. This disadvantage is compensated by a very complicated apparatus for introducing and distributing the oxygen, which impairs the advantage of autothermal dehydrogenation.
The European patent application EP 09 177 649.2, which is not a prior publication, discloses a reactor and also a process for the autothermal gas-phase dehydrogenation of hydrocarbons using heterogeneous catalysts configured as monoliths, which ensure safe control of the combustible reaction media at high reaction temperatures, frequently in the range from about 400 to 700° C., and also easy accessibility and handling of the monoliths, in particular on equipping the reactor and also on changing the catalyst.
EP 09 177 649.2 provides a reactor in the form of an essentially horizontal cylinder for carrying out an autothermal gas-phase dehydrogenation of a hydrocarbon-comprising gas stream by means of an oxygen-comprising gas stream to give a reaction gas mixture over a heterogeneous catalyst configured as monolith, wherein                the interior of the reactor is divided by a detachable, cylindrical or prismatic housing G which is arranged in the longitudinal direction of the reactor and is gastight in the circumferential direction and open at two end faces of the housing into        an inner region A having one or more catalytically active zones, in which a packing composed of monoliths stacked on top of one another, next to one another and behind one another and before each catalytically active zone in each case a mixing zone having solid internals are provided, and        an outer region B arranged coaxially to the inner region A,        with one or more feed lines for the hydrocarbon-comprising gas stream to be dehydrogenated into the outer region B, deflection of the hydrocarbon gas stream to be dehydrogenated at one end of the reactor and introduction via a flow equalizer into the inner region A,        with one or more feed lines which can be regulated independently of one another, where each feed line supplies one or more distribution chambers for the oxygen-comprising gas stream into each of the mixing zones and        with a discharge line for the reaction mixture of the autothermal gas-phase dehydrogenation at the same end of the reactor as the feed line for the hydrocarbon gas stream to be dehydrogenated.        
At the end of the reactor at which the discharge line for the reaction gas mixture from the autothermal gas-phase dehydrogenation is arranged, it is advantageous to provide a shell-and-tube heat exchanger having a bundle of tubes through which the reaction gas mixture from the autothermal gas-phase dehydrogenation is passed and also intermediate spaces between the tubes through which the hydrocarbon-comprising gas stream to be dehydrogenated is passed in countercurrent to the reaction mixture from the autothermal gas-phase dehydrogenation.
An improved reactor from a safety point of view is described in EP 10 196 216.5, namely a reactor in the form of an essentially horizontal cylinder or prism for carrying out an autothermal gas-phase dehydrogenation of a hydrocarbon-comprising gas stream by means of an oxygen-comprising gas stream to give a reaction gas mixture over a heterogeneous catalyst configured as monolith, where                the interior of the reactor is divided by a detachable, cylindrical or prismatic, gastight housing G which is arranged in the longitudinal direction of the reactor into        an inner region A having one or more catalytically active zones, in each of which a packing composed of monoliths stacked on top of one another, next to one another and behind one another and before each catalytically active zone in each case a mixing zone having solid internals are provided, and        an outer region B arranged coaxially to the inner region A, and                    a heat exchanger is provided at one end of the reactor connected to the housing G,            with one or more feed lines for the hydrocarbon-comprising gas stream to be dehydrogenated,            with one or more feed lines which can be regulated independently of one another, where each feed line supplies one or more distribution chambers, for the oxygen-comprising gas stream into each of the mixing zones and            with a discharge line for the reaction gas mixture of the autothermal gas phase dehydrogenation,                        and where the outer region B is supplied with a gas which is inert under the reaction conditions of the autothermal gas-phase dehydrogenation and the hydrocarbon-comprising gas stream to be dehydrogenated is introduced via a feed line into the heat exchanger, is heated by means of the reaction gas mixture in countercurrent by indirect heat exchange and conveyed further to the end of the reactor opposite the heat exchanger, redirected there, introduced via a flow equalizer into the inner region A and mixed with the oxygen-comprising gas stream in the mixing zones, whereupon the autothermal gas-phase dehydrogenation takes place in the inner region A of the reactor.        
Thus, a reactor having an outer reactor wall, i.e. a pressure-bearing shell which is not contacted by a medium, neither by the hydrocarbon-comprising stream nor by the oxygen-comprising stream, is provided.